Phase change memory and method of fabricating same

ABSTRACT

A fine pitch phase change random access memory (“PCRAM”) design and method of fabricating same are disclosed. One embodiment is a phase change memory (“PCM”) cell comprising a spacer defining a rectangular reaction area and a phase change material layer disposed within the reaction area. The PCM cell further comprises a protection layer disposed over the GST film layer and within the area defined by the spacer; and a capping layer disposed over the protection layer and the spacer.

BACKGROUND

Recently, alternative nonvolatile memory devices such as phase changerandom access memory (PCRAM) devices, magnetic random access memory(MRAM) devices and ferroelectric random access memory (FRAM) deviceshaving cell structures similar to those of DRAM devices, have beenproposed and are being developed. A memory cell of a PCRAM generallyincludes a phase change element comprised of a chalcogenide alloy suchas germanium antimony tellurium (“Este” or “GST”), for example, and astructure such as a transistor or other device that applies current tothe phase change element (“PCE”). In one embodiment, one source/drain ofthe transistor may be coupled to ground with the other source/draincoupled to the PCE and the transistor gate coupled to a gate voltage.Another portion of the PCE may be coupled to a bit line voltage.According to this embodiment, when the data stored within the PCE is tobe accessed, a voltage is applied to turn on the transistor and the bitline voltage is applied to the phase change material such that a readcurrent may flow through the PCE and the transistor. Based on the levelof output current, the data stored within the PCE is accessed.

Using the aforementioned arrangement or other arrangements, the level ofoutput current depends upon the phase and impedance of the phase changematerial. By changing the phase of a phase change material such as fromamorphous to crystalline or vice versa, the impedance of the phasechange material may dramatically change. The changing impedance of thephase change material enables the phase change material to storedifferent data. For example, the low-impedance form of the phase changematerial may store a data value of “1” whereas the high impedance formof the phase change material may store a data value of “0.”

There is ever-increasing pressure to reduce the size of the reaction, orcontact, area of PCRAM cells. This is due to both increasing pressure toreduce the overall size of the cell, as well as the fact that a smallerreaction area results in a faster memory cell. Additionally, the processcurrently used to fabricate PCRAM cells can induce voids in the GSTlayer due to poor sealing layer coverage resulting in outgassing of theGST at high temperatures. In particular, poor step coverage will inducevoids in the GST layer and will worsen outgassing when high temperatures(e.g., greater than 250° C.) are applied at the back end of thefabrication process.

Based on the foregoing, what is needed are PCRAM structures and methodsfor fabricating same that have smaller reaction areas and that do notsuffer the GST void-inducing outgassing problems prevalent in currentfabrication methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 illustrates an embodiment of a phase change memory (“PCM”) cell.

FIG. 2 is a graph showing a relationship between PCM cell reaction areaand reset current.

FIGS. 3A-3D collectively illustrate a method of fabricating a PCM cellin accordance with prior art embodiments.

FIGS. 4A and 4B collectively illustrate a PCM cell arrangement inaccordance with embodiments described herein.

FIGS. 5A-5F collectively illustrate a method of fabricating a PCM cellin accordance with embodiments described herein.

FIG. 6 illustrates a comparison between a PCM cell fabricated inaccordance with the method of FIGS. 3A-3D and a PCM cell fabricated inaccordance with the method of FIGS. 5A-5F.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

FIG. 1 illustrates a PCRAM cell 100 according to one or more embodimentsof the present disclosure. The PCRAM cell 100 includes a reaction area,or contact area, generally designated by a reference numeral 102. Thereaction area correlates to a reset current of the PCRAM cell 100.Referring to FIG. 2, a graph 200 illustrates an example correlationbetween the reaction area (in nm²) of a PCRAM cell and the reset current(in mA). As is apparent from the graph 200, as the size of the reactionarea of a PCRAM cell increases, so does the corresponding reset current;therefore, one way to reduce the amount of current required to reset thecell is to decrease its reaction area. Other pressure to reduce the sizeof the reaction area originates from the fact that the criticaldimension (“CD”) for an N65 generation PCRAM is 50 nm; the CD for thenext generation (N45) PCRAM will be 30 nm.

Additionally, as previously noted, GST films easily outgases attemperatures higher than 250° C.; therefore, poor step coverage causedby prior art fabrication techniques will induce voids in the GST layerand worsen outgassing when higher temperatures are applied at the backend during deposition of other layers, such as ILD and LK.

FIGS. 3A-3D illustrate a conventional PCRAM cell, and a method offabrication, represented in the figures by a cell portion 300. FIG. 3Aillustrates the cell portion 300 comprising a pair of electrodes 302,which may comprise tungsten (“W”), disposed on an interlayer dielectric(“ILD”) layer 303, after the surface of the cell portion has beenplanarized via chemical mechanical polish (“CMP”) process, for example.FIG. 3B illustrates the cell portion 300 after a phase change materiallayer 304 and a hard mask (“HM”) 306 have been deposited in aconventional manner. In one embodiment, the phase change material layer304 comprises a germanium antimony tellurium (“Este” or “GST”) filmhaving a thickness of approximately 200 {acute over (Å)}, while the HMlayer 306 comprises a silicon nitride (“SiN”) layer 306A having athickness of approximately 300 {acute over (Å)}, an oxide layer 306Bhaving a thickness of approximately 200 {acute over (Å)}, and a siliconoxynitride (“SiON”) layer 306C having a thickness of approximately 400{acute over (Å)}.

FIG. 3C illustrates the cell portion 300 after the layers 304, 306A,306B, 306C, have been etched to form a reaction area 307. Finally, FIG.3D illustrates the cell portion 300 after deposition of a sealing layer308, comprising an SiN layer having a thickness of approximately 400{acute over (Å)}.

FIGS. 4A-4B illustrate an arrangement 400 with a embodiment of a PCRAMcell 402 fabricated in accordance with features of this disclosuredescribed herein. FIG. 4A illustrates a top view of the arrangement 400of the PCRAM cell 402. FIG. 4B is a cross-sectional view obtained fromthe vertical plane along X-direction in FIG. 4A. FIGS. 5A-5F illustratefabrication of the cell 402. As shown in FIGS. 4A and 4B, a pair ofelectrodes 404A, 404B, which may be comprised of W, are disposed in anILD 406. As best illustrated in FIG. 4A, a first conductor 408A isdisposed beneath the electrode 404A, while a second conductor 408B isdisposed above the electrode 404B in a manner that is offset from thecell 402.

FIG. 5A illustrates the cell 402 after deposition of a hard mask (HM)500, which comprises a SiN layer 500A having a thickness of between 100and 500 {acute over (Å)}. In one embodiment, the thickness isapproximately 300 {acute over (Å)}. The HM 500 also includes an oxidelayer 500B having a thickness of between 100 and 500 {acute over (Å)}.In one embodiment, the thickness is approximately 200 {acute over (Å)}.The HM 500 also includes a SiON layer 500C having a thickness of between100 and 500 {acute over (Å)}. In one embodiment, the thickness isapproximately 400 {acute over (Å)}. The opening 501A in the HM 500 isformed by a lithography process, after which is performed a dielectricetch process. The remaining HM 500 defines the outer edge of the cell402. The opening 501A exposes portions of the electrodes 404A and 404Band the ILD layer 106 between the electrodes 404A and 404B.

FIG. 5B illustrates the cell 402 after a spacer layer 504 has beendeposited over the opening 501A and the remaining HM 500. In oneembodiment, the spacer layer 504 comprises one of SiN, SiO₂, or SiONhaving a thickness in the range of approximately 100 {acute over (Å)} to500 {acute over (Å)}, depending on the CD of the cell 402, and defines areaction area 505 of the cell 402.

FIG. 5C illustrates the cell 402 after isotropic etching of the spacerlayer 504 to create a spacer 506 along the sidewalls of the remaining HM500. The spacer 506 covers a portion of the exposed the electrodes 404Aand 404B shown in FIG. 5A. The spacer 506 serves the purpose of reducingthe dimension of the reaction area without adding lithography upgrade.The opening 501A is then further shaped to create a smaller opening501B. In the present embodiment, the width of the spacer 506 is fairlyclose to the thickness of the spacer layer 504; i.e., in the range ofapproximately 100 {acute over (Å)} to 500 {acute over (Å)}.

FIG. 5D illustrates the cell 402 after deposition of a phase changematerial (PCM) layer 508, which in one embodiment comprises a GST(GeSbTe) film. The phase change material layer 508 is formed over theopening 501B, the spacer 506, and the remaining HM 500. The GST film mayhave a thickness of between 100 and 500 {acute over (Å)}. In oneembodiment, the thickness is approximately 200 {acute over (Å)}. Aprotection layer 510 is deposited on top of PCM layer 508. Theprotection layer may comprise SiN or SiO₂ having a thickness in therange of approximately 200 {acute over (Å)} to 500 {acute over (Å)}.

FIG. 5E illustrates the cell 402 following a performance of a chemicalmechanical polishing (CMP) process, which is controlled to stop on oneof the HM layers, i.e., the SiN layer 500A. The depth of CMP polishingis determined by the remaining thickness of SiN, which in one embodimentis approximately 500 {acute over (Å)}. After the CMP process, the cell402 has a planarized topography. The reaction area 505 of the cell 402is defined by the spacer 506. The PCM layer 508 is disposed within thereaction area 505. The protection layer 510 is disposed over the PCMlayer 508 and within the reaction area 505 defined by the spacer 506.

FIG. 5F illustrates the cell 402 after a capping layer 512 is depositedon top of planarized cell 402 to establish a top seal for the cell. Thecapping layer 512 may comprise SiN. The thickness of the capping layer512 may be between about 400 {acute over (Å)} and 1000 {acute over (Å)}.The thickness of the capping layer may be selected to ensure sufficientencapsulation, but still allow for a wide process window.

A PCRAM cell fabricated in accordance with the method illustrated inFIGS. 5A-5F, such as the cell 402, may be fabricated to achieverelatively small critical dimensions without requiring advancedlithography processes and/or tools to shrink the CD of the PCRAM cell.Moreover, the method illustrated in FIGS. 5A-5F avoids the step coverageproblem, especially at foot of the steps, which results in induction ofGST void and outgassing during the high temperature processes that mayoccur during other processes.

FIG. 6 illustrates a comparison between a PCM cell fabricated inaccordance with the method illustrated in FIGS. 3A-3D, represented by acell 600, and a PCM cell fabricated in accordance with them methodillustrated in FIGS. 5A-5F, represented by a cell 602. In particular, asshown in FIG. 6, in cell 600 the reaction area structure is formed as astep shape, which presents the challenge of providing good step coverageby the capping layer. This is primarily due to GST outgassing thatoccurs during the subsequent high temperature process. The poor cappinglayer coverage at the foot of the step 604 is of particular concern. Incontrast, in the cell 602 the reaction area is formed to have a planartopography, thereby insuring good capping layer coverage. Moreover, thedimension of the actual reaction area of the cell 602 is reduced, due tothe addition of the spacer structure within the reaction area.

Accordingly, the present disclosure describes a phase chang memory(“PCM”) cell. In one embodiment, the PCM cell comprises a spacerdefining a rectangular reaction area and a phase change material layerdisposed within the reaction area. The PCM cell further comprises aprotection layer disposed over the GST film layer and within the areadefined by the spacer; and a capping layer disposed over the protectionlayer and the spacer.

The present disclosure also describes a method of fabricating a phasechange memory (“PCM”) device comprising an interlayer dielectric (“ILD”)layer having electrodes disposed at opposite ends thereof. The methodcomprises defining a first rectangular area on a top surface of the ILDlayer using a hard mask; creating a spacer along an inner edge of thehard mask layer and an outer edge of the first rectangular area todefine a second rectangular area; and depositing a phase change materiallayer within the second rectangular area. The method further comprisesdepositing a protection layer over the phase change material layer;performing a chemical mechanical polish (“CMP”) process to isolate thephase change material layer; and depositing a capping layer over thedevice.

Yet another embodiment is a method of fabricating a phase change memory(“PCM”) device comprising an interlayer dielectric (“ILD”) layer havingelectrodes disposed at opposite ends thereof. The method comprisesdefining a first rectangular area on a top surface of the ILD layerusing a hard mask by depositing at least one layer comprising the hardmask on the ILD layer; and etching the hard mask to create the firstrectangular area, wherein the at least one layer comprising the hardmask comprises an outer boundary of the first rectangular area. Themethod further comprises creating a spacer along an inner edge of thehard mask layer and an outer edge of the first rectangular area todefine a second rectangular area by depositing a spacer layer; andetching the spacer layer to create the spacer. The method furthercomprises depositing a phase change material layer within the secondrectangular area; depositing a protection layer over the phase changematerial layer; performing a CMP process on a top surface of the deviceto isolate the phase change material layer, the method furthercomprising stopping the CMP process a designated layer of the HM; anddepositing a capping layer over the device.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. In particular, different materials may besubstituted for the materials mentioned herein; for example,chalcogenide films other than GST may be used to implement the phasechange material.

It is understood that various different combinations of the above-listedembodiments and steps can be used in various sequences or in parallel,and there is no particular step that is critical or required. Moreover,each of the modules depicted in the drawings can be implemented onmultiple devices, including computing devices, and implementation ofmultiple ones of the depicted modules may be combined into a singledevice, including a computing device. Furthermore, features illustratedand discussed above with respect to some embodiments can be combinedwith features illustrated and discussed above with respect to otherembodiments. Accordingly, all such modifications are intended to beincluded within the scope of this invention.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A phase change memory (“PCM”) cell comprising: a spacer defining areaction area; a phase change material (PCM) layer disposed within thereaction area; a protection layer disposed over the PCM layer and withinthe reaction area defined by the spacer; and a capping layer disposedover the protection layer and the spacer.
 2. The PCM cell of claim 1further comprising an electrode disposed underneath one end of thereaction area.
 3. The PCM cell of claim 1 further comprising a stoplayer disposed along an outer edge of the spacer.
 4. The PCM cell ofclaim 1 wherein the spacer comprises at least one of SiN, SiO₂, and SiONand has a thickness in a range of approximately 100 {acute over (Å)}-500{acute over (Å)}.
 5. The PCM cell of claim 1 wherein the phase changematerial comprises a GeSbTe alloy film.
 6. The PCM cell of claim 1wherein the protection layer comprises one of SiN and SiO₂ and has athickness in a range of 200 {acute over (Å)}-500 {acute over (Å)}. 7.The PCM cell of claim 1 wherein the capping layer comprises SiN and hasa thickness in a range of 400 {acute over (Å)}-1000 {acute over (Å)}. 8.The PCM cell of claim 1 further comprising a conductor disposed belowand in contact with the electrode.
 9. A method of fabricating a phasechange memory (“PCM”) device comprising an interlayer dielectric (“ILD”)layer having electrodes disposed at opposite ends thereof, the methodcomprising: defining a first area on a top surface of the ILD layerusing a hard mask; creating a spacer along an inner edge of the hardmask layer and an outer edge of the first area to define a second area;depositing a phase change material layer within the second area;depositing a protection layer over the phase change material layer;isolating the phase change material layer; and depositing a cappinglayer over the device.
 10. The method of claim 9 wherein the definingcomprises: depositing at least one layer comprising the hard mask on theILD layer; and etching the hard mask to create the first area, whereinthe at least one layer comprising the hard mask comprises an outerboundary of the first area.
 11. The method of claim 10 wherein thedepositing at least one layer comprising the hard mask comprisesdepositing at least one of an SiN layer, an oxide layer, and an SiONlayer over the ILD layer, such that a total thickness of the hard masklayer is in a range of approximately 500 {acute over (Å)}-2000 {acuteover (Å)}.
 12. The method of claim 10 wherein the depositing at leastone layer comprising the hard mask comprises: depositing an SiN layer onthe ILD layer, the SiN layer having a thickness of approximately 100{acute over (Å)}-500 {acute over (Å)}; depositing an oxide layer on theSiN layer, the oxide layer having a thickness of approximately 100{acute over (Å)}-500 {acute over (Å)}; and depositing an SiON layer onthe oxide layer, the SiON layer having a thickness of approximately 200{acute over (Å)}-500 {acute over (Å)}.
 13. The method of claim 9 whereinthe creating a spacer comprises: depositing a spacer layer; and etchingthe spacer layer to create the spacer.
 14. The method of claim 13wherein the spacer comprises one of SiN, SiO₂, and SiON and has athickness of approximately 100 {acute over (Å)}-500 {acute over (Å)}.15. The method of claim 9 wherein the isolating the phase changematerial layer comprises performing a chemical mechanical polish (“CMP”)process on a top surface of the device, the method further comprisingstopping the CMP process at a designated layer of the hard mask.
 16. Themethod of claim 9 wherein the depositing a protection layer comprisesdepositing a layer of SiN or SiO₂ having a thickness of approximately200 {acute over (Å)}-500 {acute over (Å)}.
 17. The method of claim 9wherein the depositing a capping layer comprises depositing a layer ofSiN having a thickness of approximately 400 {acute over (Å)}-1000 Å 18.A method of fabricating a phase change memory (“PCM”) device comprisingan interlayer dielectric (“ILD”) layer having electrodes disposed atopposite ends thereof, the method comprising: defining a firstrectangular area on a top surface of the ILD layer using a hard mask,the defining comprising: depositing at least one layer comprising thehard mask on the ILD layer; and etching the hard mask to create thefirst rectangular area, wherein the at least one layer comprising thehard mask comprises an outer boundary of the first rectangular area;creating a spacer along an inner edge of the hard mask layer and anouter edge of the first rectangular area to define a second rectangulararea, the creating comprising: depositing a spacer layer; and etchingthe spacer layer to create the spacer; depositing a phase changematerial layer within the second rectangular area; depositing aprotection layer over the phase change material layer; performing a CMPprocess on a top surface of the device to isolate the phase changematerial layer, the method further comprising stopping the CMP processat a designated layer of the hard mask; and depositing a capping layerover the device.
 19. The method of claim 18 wherein the depositing atleast one layer comprising the hardmask comprises: depositing an SiNlayer on the ILD layer, the SiN layer having a thickness ofapproximately 3×10² {acute over (Å)}; depositing an oxide layer on theSiN layer, the oxide layer having a thickness of approximately 2×10²{acute over (Å)}; and depositing an SiON layer on the oxide layer, theSiON layer having a thickness of approximately 4×10² {acute over (Å)};wherein the designated layer is the SiN layer.
 20. The method of claim18 wherein the spacer comprises one of SiN, SiO₂, and SiON and has athickness of approximately 100 {acute over (Å)}-500 {acute over (Å)}.